Clinical use of diodes and micro-chambers to obtain accurate small field output factor measurements

  • T. KairnEmail author
  • P. H. Charles
  • G. Cranmer-Sargison
  • S. B. Crowe
  • C. M. Langton
  • D. I. Thwaites
  • J. V. Trapp
Educational Note


There have been substantial advances in small field dosimetry techniques and technologies, over the last decade, which have dramatically improved the achievable accuracy of small field dose measurements. This educational note aims to help radiation oncology medical physicists to apply some of these advances in clinical practice. The evaluation of a set of small field output factors (total scatter factors) is used to exemplify a detailed measurement and simulation procedure and as a basis for discussing the possible effects of simplifying that procedure. Field output factors were measured with an unshielded diode and a micro-ionisation chamber, at the centre of a set of square fields defined by a micro-multileaf collimator. Nominal field sizes investigated ranged from 6 × 6 to 98 × 98 mm2. Diode measurements in fields smaller than 30 mm across were corrected using response factors calculated using Monte Carlo simulations of the diode geometry and daisy-chained to match micro-chamber measurements at intermediate field sizes. Diode measurements in fields smaller than 15 mm across were repeated twelve times over three separate measurement sessions, to evaluate the reproducibility of the radiation field size and its correspondence with the nominal field size. The five readings that contributed to each measurement on each day varied by up to 0.26  %, for the “very small” fields smaller than 15 mm, and 0.18 % for the fields larger than 15 mm. The diode response factors calculated for the unshielded diode agreed with previously published results, within uncertainties. The measured dimensions of the very small fields differed by up to 0.3 mm, across the different measurement sessions, contributing an uncertainty of up to 1.2 % to the very small field output factors. The overall uncertainties in the field output factors were 1.8 % for the very small fields and 1.1 % for the fields larger than 15 mm across. Recommended steps for acquiring small field output factor measurements for use in radiotherapy treatment planning system beam configuration data are provided.


Dosimetry Solid state Radiation therapy 



Experimental measurements were obtained with assistance from Greg Pedrazzini, Richard Knight, George Warr and Trent Aland. Information and advice on early aspects of this work were provided by John Kenny. This study was supported by the Australian Research Council, the Wesley Research Institute, Premion and the Queensland University of Technology (QUT), through linkage Grant No. LP110100401.


  1. 1.
    Taylor ML, Kron T, Franich RD (2011) A contemporary review of stereotactic radiotherapy: inherent dosimetric complexities and the potential for detriment. Acta Oncol 50(4):483–508PubMedCrossRefGoogle Scholar
  2. 2.
    Das IJ, Ding GX, Ahnesjö A (2008) Small fields: nonequilibrium radiation dosimetry. Med Phys 35(1):206–215PubMedCrossRefGoogle Scholar
  3. 3.
    McKerracher C, Thwaites DI (1999) Assessment of new small-field detectors against standard-field detectors for practical stereotactc beam data acquisition. Phys Med Biol 44(9):21432160CrossRefGoogle Scholar
  4. 4.
    Francescon P, Cora S, Satariano N (2011) Calculation of kQclin, Qmsrfclin, fmsr for several small detectors and for two linear accelerators using Monte Carlo simulations. Med Phys 38(12):6513–6527PubMedCrossRefGoogle Scholar
  5. 5.
    Cranmer-Sargison G, Weston S, Evans JA, Sidhu NP, Thwaites DI (2011) Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors. Med Phys 38(12):6592–6602PubMedCrossRefGoogle Scholar
  6. 6.
    Cranmer-Sargison G, Weston S, Sidhu NP, Thwaites DI (2011) Experimental small field 6MV output ratio analysis for various diode detector and accelerator combinations. Radiother Oncol 100(3):429–435PubMedCrossRefGoogle Scholar
  7. 7.
    Ralston A, Liu P, Warrener K, McKenzie D, Suchowerska N (2012) Small field diode correction factors derived using an air core fibre optic scintillation dosimeter and EBT2 film. Phys Med Biol 57(9):2587–2602PubMedCrossRefGoogle Scholar
  8. 8.
    Bassinet C, Huet C, Derreumaux S, Brunet G, Chea M, Baumann M, Lacornerie T, Gaudaire-Josset S, Trompier F, Roch P, Boisserie G, Clairand I (2013) Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife and linear accelerators equipped with microMLC and circular cones. Med Phys 40(7):071725PubMedCrossRefGoogle Scholar
  9. 9.
    Cranmer-Sargison G, Charles PH, Trapp JV, Thwaites DI (2013) A methodological approach to reporting corrected small field relative outputs. Radiother Oncol 109(3):350–355PubMedCrossRefGoogle Scholar
  10. 10.
    Charles PH, Cranmer-Sargison G, Thwaites DI, Crowe SB, Kairn T, Knight RT, Kenny J, Langton CM, Trapp JV (2014) A practical and theoretical definition of very small field size for radiotherapy output factor measurements. Med Phys 41(4):041707PubMedCrossRefGoogle Scholar
  11. 11.
    Alfonso R, Andreo P, Capote R, Huq MS, Kilby W, Kjäll P, Mackie TR, Palmans H, Rosser K, Seuntjens J, Ullrich W, Vatnitsky S (2008) A new formalism for reference dosimetry of small and nonstandard fields. Med Phys 35(11):5179–5186PubMedCrossRefGoogle Scholar
  12. 12.
    Das IJ, Cheng C-W, Watts RJ, Ahnesjö A, Gibbons J, Li XA, Lowenstein J, Mitra RK, Simon WE, Zhu TC (2008) Accelerator beam data commissioning equipment and procedures: report of the TG- 106 of the Therapy Physics Committee of the AAPM. Med Phys 35(9):4186–4215PubMedCrossRefGoogle Scholar
  13. 13.
    Cranmer-Sargison G, Weston S, Evans JA, Sidhu NP, Thwaites DI (2012) Monte Carlo modelling of diode detectors for small field MV photon dosimetry: detector model simplification and the sensitivity of correction factors to source parameterization. Phys Med Biol 57(16):5141–5153PubMedCrossRefGoogle Scholar
  14. 14.
    Dieterich S, Sherouse GW (2011) Experimental comparison of seven commercial dosimetry diodes for measurement of stereotactic radiosurgery cone factors. Med Phys 38(7):4166–4173PubMedCrossRefGoogle Scholar
  15. 15.
    Griessbach I, Lapp M, Bohsung J, Gademann G, Harder D (2005) Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beams. Med Phys 32(12):3750–3754PubMedCrossRefGoogle Scholar
  16. 16.
    Scott AJ, Kumar S, Nahum AE, Fenwick JD (2012) Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields. Phys Med Biol 57(14):4461–4476PubMedCrossRefGoogle Scholar
  17. 17.
    Laub WU, Wong T (2003) The volume effect of detectors in the dosimetry of small fields used in IMRT. Med Phys 30(3):341–347PubMedCrossRefGoogle Scholar
  18. 18.
    Charles PH, Crowe SB, Kairn T, Knight RT, Hill B, Kenny J, Langton CM, Trapp JV (2013) Monte Carlo-based diode design for correction-less small field dosimetry. Phys Med Biol 58(13):4501–4512PubMedCrossRefGoogle Scholar
  19. 19.
    Charles PH, Crowe SB, Kairn T, Kenny J, Lehmann J, Lye J, Dunn L, Hill B, Knight RT, Langton CM, Trapp JV (2012) The effect of very small air gaps on small field dosimetry. Phys Med Biol 57(21):6947–6960PubMedCrossRefGoogle Scholar
  20. 20.
    Pappas E, Maris TG, Zacharopoulou F, Papadakis A, Manolopoulos S, Green S, Wojnecki C (2008) Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison. Med Phys 35(10):4640–4648PubMedCrossRefGoogle Scholar
  21. 21.
    Yorke E, Alecu R, Ding L, Fontenla D, Kalend A, Kaurin D, Masterson-McGary M E, Marinello G, Matzen T, Saini A, Shi J, Simon W, Zhu T C, Zhu X R (2005) Diode in vivo dosimetry for patients receiving external beam radiation therapy: Report of Task Group 62 of the Radiation Therapy Committee. American Association of Physicists in MedicineGoogle Scholar
  22. 22.
    Zhu XR, Allen JJ, Shi J, Simon WE (2000) Total scatter factors and tissue maximum ratios for small radiosurgery fields: comparison of diode detectors, a parallel-plate ion chamber, and radiographic film. Med Phys 27(3):472–477PubMedCrossRefGoogle Scholar
  23. 23.
    Beddar AS, Mason DJ, O’Brien PF (1994) Absorbed dose perturbation caused by diodes for small field photon dosimetry. Med Phys 21(7):1075–1079PubMedCrossRefGoogle Scholar
  24. 24.
    Charles PH, Cranmer-Sargison G, Thwaites DI, Kairn T, Crowe SB, Pedrazzini G, Aland T, Kenny J, Langton CM, Trapp JV (2014) Design and experimental testing of air slab caps which convert commercial electron diodes into dual purpose, correction-free diodes for small field dosimetry. Med Phys 41(10):101701PubMedCrossRefGoogle Scholar
  25. 25.
    Rogers DWO, Faddegon BA, Ding GX, Ma CM, We J, Mackie TR (1995) BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Med Phys 22(5):503–524PubMedCrossRefGoogle Scholar
  26. 26.
    Francescon P, Kilby W, Satariano N, Cora S (2012) Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system. Phys Med Biol 57(12):3741–3758PubMedCrossRefGoogle Scholar
  27. 27.
    Pantelis E, Moutsatsos A, Zourari K, Petrokokkinos L, Sakelliou L, Kilby W, Antypas C, Papagiannis P, Karaiskos P, Georgiou E, Seimenis I (2012) On the output factor measurements of the CyberKnife iris collimator small fields: experimental determination of the kQ clin, Q msr f clin, f msr correction factors for microchamber and diode detectors. Med Phys 39(8):4875–4885PubMedCrossRefGoogle Scholar
  28. 28.
    Charles PH, Crowe SB, Kairn T, Knight R, Hill B, Kenny J, Langton CM, Trapp JV (2014) The influence of Monte Carlo source parameters on detector design and dose perturbation in small field dosimetry. J Phys Conf Ser 489:012006CrossRefGoogle Scholar
  29. 29.
    Kawrakow I (2005) egspp: the EGSnrc C++ class library. NRCC Report PIRS-899, National Research Council of CanadaGoogle Scholar
  30. 30.
    Kawrakow I, Rogers DWO, Walters BRB (2004) Large efficiency improvements in BEAMnrc using directional bremsstrahlung splitting. Med Phys 31(10):2883–2898PubMedCrossRefGoogle Scholar
  31. 31.
    Kairn T, Kenny J, Crowe SB, Fielding AL, Franich RD, Johnston PN, Knight R, Langton CM, Schlect D, Trapp JV (2010) Technical note: modelling a complex micro-multileaf collimator using the standard BEAMnrc distribution. Med Phys 37(4):1761–1767PubMedCrossRefGoogle Scholar
  32. 32.
    Kairn T, Aland T, Franich RD, Johnston PN, Kakakhel MB, Kenny J, Knight R, Langton CM, Schlect D, Taylor ML, Trapp JV (2010) Adapting a generic BEAMnrc model of the BrainLAB m3 micro-multileaf collimator to simulate a local collimation device. Phys Med Biol 55(17):N451–N463PubMedCrossRefGoogle Scholar
  33. 33.
    Kairn T, Taylor ML, Crowe SB, Dunn L, Franich RD, Kenny J, Knight RT, Trapp JV (2012) Monte Carlo verification of gel dosimetry measurements for stereotactic radiotherapy. Phys Med Biol 57(11):3359–3369PubMedCrossRefGoogle Scholar
  34. 34.
    Brainlab AG (2010) Brainlab Physics Technical Guide, Revision 1.2Google Scholar
  35. 35.
    ISO. Guide to expression of uncertainty in measurement. Technical Report Guide 98 (International Organization of Standardization, Geneva, 1995)Google Scholar
  36. 36.
    Hill R (2013) Reporting uncertainties in measurement: what approach should be followed? Australas. Phys Eng Sci Med 36(1):1–3PubMedCrossRefGoogle Scholar
  37. 37.
    Scott AJ, Nahum AE, Fenwick JD (2009) Monte Carlo modeling of small photon fields: quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small-field measurements. Med Phys 36(7):3132–3144PubMedCrossRefGoogle Scholar
  38. 38.
    Li S, Rashid A, He S, Djajaputra D (2004) A new approach in dose measurement and error analysis for narrow photon beams (beamlets) shaped by different multileaf collimators using a small detector. Med Phys 31(7):2020–2032PubMedCrossRefGoogle Scholar
  39. 39.
    Tyler M, Liu PZ, Chan KW, Ralston A, McKenzie DR, Downes S, Suchowerska N (2013) Characterization of small-field stereotactic radiosurgery beams with modern detectors. Phys Med Biol 58(21):7595–7608PubMedCrossRefGoogle Scholar
  40. 40.
    Liu PZ, Suchowerska N, McKenzie DR (2014) Can small field diode correction factors be applied universally? Radiother Oncol 112(3):442–446Google Scholar
  41. 41.
    Kairn T, Asena A et al (2015) Field size consistency of nominally matched linacs. Australas. Phys Eng Sci Med (Submitted)Google Scholar

Copyright information

© Australasian College of Physical Scientists and Engineers in Medicine 2015

Authors and Affiliations

  • T. Kairn
    • 1
    • 2
    Email author
  • P. H. Charles
    • 2
    • 3
  • G. Cranmer-Sargison
    • 4
    • 5
  • S. B. Crowe
    • 2
    • 7
  • C. M. Langton
    • 2
  • D. I. Thwaites
    • 6
  • J. V. Trapp
    • 2
  1. 1.Genesis CancerCare QueenslandBrisbaneAustralia
  2. 2.Queensland University of TechnologyBrisbaneAustralia
  3. 3.Radiation OncologyPrincess Alexandra HospitalBrisbaneAustralia
  4. 4.Department of Medical PhysicsSaskatchewan Cancer AgencySaskatoonCanada
  5. 5.Faculty of Medicine and HealthUniversity of LeedsLeedsUnited Kingdom
  6. 6.Institute of Medical PhysicsUniversity of SydneyCamperdownAustralia
  7. 7.Cancer Care ServicesRoyal Brisbane and Women’s HospitalBrisbaneAustralia

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